Cellulose hydrate ultrafiltration membranes are usually produced by saponification from cellulose ester ultrafiltration membranes. Cellulose ester ultrafiltration membranes are made mostly of cellulose acetates, such as cellulose diacetate and cellulose triacetate; however, they can be also produced from cellulose propionates, cellulose butyrates, cellulose acetobutyrates and mixtures thereof.
Unlike membranes produced from cellulose esters or from synthetic polymers, the advantages of cellulose hydrate membranes exhibit high solvent resistance, higher temperature stability and lower nonspecific adsorption. These advantageous characteristics of cellulose hydrate membranes can be offset in certain applications by the disadvantage comprising relatively expensive production. The characteristics of the membranes are altered by saponification, i.e., they are modified to higher “molecular weight cut offs” (hereinafter abbreviated as “cut off” or “c.o.”), a term of art used in membrane technology to indicate a membrane's retention capability. A higher cut off number means that the membrane has a lower capability to retain substances having a lower molecular weight. In general, there is no simple relationship between the cut off of the starting cellulose ester membrane (for example, a cellulose acetate membrane) and the cut off of the cellulose hydrate membrane created by saponification of the starting membrane.
A manifestation of this general rule is that, under identical saponification conditions, cellulose acetate membranes having a given cut off can yield cellulose hydrate membranes having different cut offs, depending on the manner in which the cellulose acetate membrane was manufactured. A general rule is that the ratio of flux to cut off of membranes manufactured from cellulose acetate and from synthetic polymers is significantly more advantageous than that of membranes manufactured from cellulose hydrates; this is the main obstacle to obtaining lower cut offs for cellulose hydrate ultrafiltration membranes.
In particular, in the manufacture of small-pored cellulose hydrate ultrafiltration membranes having an approximate cut off in the range of 50 Daltons (abbreviated as Da) and 10 kDa, it is particularly difficult to reliably adjust the porosity of the membrane. A reliable adjustment of the porosity can be achieved only at the level of relatively low through-flow or flux rates, when for example very high polymer concentration and/or a smaller amount of a swelling agent are used in the casting solution during the production of the starting cellulose ester membranes. These measures make it possible to reduce the cut off of the starting cellulose acetate membranes to such an extent that saponification achieves the desired cut off. However, in addition to an unfavorable flux:rejection ratio, another disadvantage of this method is that the cut off of the resulting cellulose hydrate membrane is predetermined by the makeup of the casting solution. And when the starting cellulose acetate membranes are produced in separate machines, which is usually the case, this method is expensive because storage of the cellulose acetate membranes is required.
According to another known prior art method (EP 0 762 929 B1) of making cellulose hydrate ultrafiltration membranes, the membranes are crosslinked following saponification of the cellulose ester membranes. Although such membranes have greater chemical stability, in particular in relation to alkaline compounds and cellulose-decomposing enzymes (cellulases), different cut offs are nevertheless obtained from identical cellulose acetate starting membranes, depending on the degree of crosslinking. However, the flux:retention ratio is usually no more favorable than when crosslinking is not employed.
Accordingly, a principal object of the invention is to provide a cellulose hydrate ultrafiltration membrane which has improved filtration characteristics, in particular improved flux for a given cut off class.